The present invention relates to sulphonic fluorinated ionomers suitable for the preparation of membranes working from room temperature to high temperatures, of the order of 120xc2x0 C.-180xc2x0 C., in electrolytic applications, for example in fuel cells.
Specifically, the invention relates to sulphonic fluorinated ionomers crosslinked without involving the xe2x80x94SO2F groups and capable to maintain a high degree of hydration, both at room and at high temperature (up to 120xc2x0-180xc2x0 C.), without substantially compromising the physical integrity of the membrane.
More specifically, in the case of crosslinked sulphonic fluorinated ionomers and having a low equivalent weight, lower than about 750, the obtained membranes show a high capability of water absorption, both at room and at high temperature (up to 120xc2x0-180xc2x0 C.), without substantially compromising the physical integrity of the membrane.
In the case of sulphonic fluorinated ionomers having an equivalent weight higher than about 750 and up to about 1,300, by the crosslinking of the invention it is possible to prepare membranes having an extremely thin thickness, for example in the range 10-80 xcexcm, which maintain a good hydration also at high temperatures, of the order of 120xc2x0 C.-180xc2x0 C., still maintaining the physical integrity.
It is known in the prior art the use of the class of polymers called by the term xe2x80x9cionomersxe2x80x9d in electrochemical applications, such as for example in fuel cells, chloro-alkali cells, lithium batteries, electrodialysis and in reactors in which the ionomer acts as a solid catalyst. These applications imply the contact of the ionomer with an aqueous or polar liquid having affinity with the ionic functional groups of the ionomer.
Generally, the larger the amount of sulphonic groups (ionomers having a low equivalent weight), the better the efficiency of the ionomer in the application, both in terms of ion exchange capability in electrochemical applications, and in terms of the catalyst activity in catalysis applications. From this point of view, an important parameter is the equivalent weight of the ionomer. The lower the equivalent weight, the higher the percentage of ionic groups. Therefore, ionomers having a low equivalent weight are desirable since they give a higher efficiency in the application.
In electrochemical applications, for example in fuel cells, there is a direct correlation between the ionomer conductivity and the retention of water of the ionomer. The ionic conductivity of the polymer, besides being increased by the higher presence of ionic groups in the polymer, results increased, within certain limits, also by the larger amount of water that the polymer is capable to keep (swelling degree). However, the excessive affinity of the ionomer with water has the drawback of an excessive swelling of the ionomer, which takes a gelatinous state thus losing its physical integrity. The ionomer therefore becomes completely unusable in all the applications wherein it is required under a solid form.
Also in the applications wherein the ionomer is mixed with or deposited on a support material, suitable to guarantee the form and the physical integrity of the final membrane, the ionomer must however show a sufficient physical consistency to prevent the release from the support and it must be quite insoluble in water with which it comes into contact during the use. Besides, the ionomer/membrane must be activated before the use, wherefore the chemical transformation of the precursor groups xe2x80x94SO2F into the corresponding ionic groups xe2x80x94SO3H is necessary. The membrane activation is carried out first by contacting it with an alkaline aqueous solution and then with an acid solution. During this transformation phase, if the ionomer has a high swelling degree, it can partially or completely dissolve in the reaction medium. At this point, it is extremely difficult to recover the ionomer and separate it from the by-products of the transformation reaction.
In the prior art, to obtain a limited hydration of the ionomer and sufficient physical integrity, polymers having a high equivalent weight, of the order of 1,000-1,200, i.e. having a low concentration of sulphonic groups, are used. Ionomers having a high equivalent weight absorb a limited amount of water, which guarantees the polymer insolubility. On the other hand, having few ionic groups, they have the drawback to give membranes with a low ionic conductivity during the application. An example of said membranes is represented by the commercial product NAFION(copyright), used in fuel cells. These membranes to have a good physical integrity must however have a high thickness, generally higher than 100 xcexcm. Besides, if these membranes are used at temperatures higher than 100xc2x0 C., the water contained in the membrane, due to the limited number of hydrophilic groups xe2x80x94SO3H and the high thickness, tends to diminish, wherefore the membrane tends to dehydrate and the membrane conductivity is drastically reduced. Consequently, the NAFION(copyright) membranes are not effectively usable at temperatures higher than 100xc2x0 C.
U.S. Pat. No. 4,940,525 describes sulphonic ionomers having a low equivalent weight, lower than 725, used to obtain unsupported thick membranes for fuel cells, only if the hydration product of the polymer is lower than 22,000. According to this patent so low hydration values are indeed necessary to maintain the polymer physical integrity at equivalent weights lower than 725, provided that the equivalent weight is not lower than 500 (col. 6, 8-16). In this patent no mention is made either to the behaviour of these membranes at high temperatures, up to about 120xc2x0 C.-180xc2x0 C., or to the minimum usable thickness maintaining the physical integrity.
The need was therefore felt to have available sulphonic fluorinated ionomers able to give membranes usable both at room and at high temperature (up to 120xc2x0-180xc2x0 C.), without substantially compromising the physical integrity of the ionomeric membrane for sulphonic fluorinated ionomers having a low equivalent weight, lower than 750; in the case of sulphonic fluorinated ionomers having an equivalent weight higher than about 750 and up to about 1,300, to have membranes having an extremely thin thickness, for example in the range 10-80 xcexcm.
The Applicant has surprisingly and unxpectedly found sulphonic fluorinated ionomers able to solve the above mentioned technical problem.
An object of the present invention are crosslinked sulphonic fluorinated ionomers, where crosslinking does not involve the xe2x80x94SO2F groups, having an equivalent weight 380-1,300 g/eq, and comprising:
(A) monomeric units deriving from one or more fluorinated monomers containing at least one ethylene unsaturation;
(B) fluorinated monomeric units containing sulphonyl groups xe2x80x94SO2F in an amount such as to give the above equivalent weight.
The fluorinated monomers of type (A) are selected from:
vinylidene fluoride (VDF);
C2-C8 perfluoroolefins, preferably tetrafluoroethylene (TFE);
C2-C8 chloro- and/or bromo- and/or iodo-fluoroolefins, such as chlorotrifluoroethylene (CTFE) and bromotrifluoroethylene;
CF2xe2x95x90CFORf (per)fluoroalkylvinylethers (PAVE), wherein Rf is a C1-C6 (per)fluoroalkyl, for example trifluoromethyl, bromodifluoromethyl, pentafluoropropyl;
CF2xe2x95x90CFOX perfluoro-oxyalkylvinylethers, wherein X is a C1-C12 perfluoro-oxyalkyl having one or more ether groups, for example perfluoro-2-propoxy-propyl.
The fluorinated monomers of type (B) are selected from one or more of the following:
F2Cxe2x95x90CFxe2x80x94Oxe2x80x94CF2xe2x80x94CF2xe2x80x94SO2F;
F2Cxe2x95x90CFxe2x80x94Oxe2x80x94[CF2xe2x80x94CXFxe2x80x94O]nxe2x80x94CF2xe2x80x94CF2xe2x80x94SO2F
wherein Xxe2x95x90Cl,F or CF3; n=1-10
F2Cxe2x95x90CFxe2x80x94Oxe2x80x94CF2xe2x80x94CF2xe2x80x94CF2xe2x80x94SO2F
F2Cxe2x95x90CFxe2x80x94Arxe2x80x94SO2F wherein Ar is an aryl ring.
Optionally the sulphonic fluorinated ionomers of the invention can contain from 0.01 to 5% by moles of monomeric units deriving from a bis-olefin of formula:
R1R2 Cxe2x95x90CHxe2x80x94(CF2)mxe2x80x94CHxe2x95x90CR5R6xe2x80x83xe2x80x83(I)
wherein:
m=2-10, preferably 4-8;
R1, R2, R5, R6, equal to or different from each other, are H or C1-C5 alkyl groups.
The introduction as comonomer of the bis-olefin of formula (I), having a number of unsaturations higher than the unit, is advantageous since said comonomer has the function to pre-crosslink the ionomer in the polymerization step. The introduction of the bis-olefin has the advantage to increase the length of the primary chains which will form the final network.
Preferably the sulphonic fluorinated ionomers of the present invention are crosslinked by peroxidic route, wherefore they must contain radical attack sites in the chain and/or in the terminal position of the macromolecules, for example iodine and/or bromine atoms.
Preferably the crosslinked fluorinated sulphonic ionomers of the invention comprise:
monomeric units deriving from TFE;
monomeric units deriving from CF2xe2x95x90CFxe2x80x94Oxe2x80x94CF2CF2SO2F;
monomeric units deriving from the bis-olefin of formula (I).
iodine atoms in terminal position.
As regards the introduction in the chain of said iodine and/or bromine atoms, it can be carried out by addition, in the reaction mixture, of brominated and/or iodinated xe2x80x9ccure-sitesxe2x80x9d comonomers, such as-bromo and/or iodo olefins having from 2 to 10 carbon atoms (as described for example in U.S. Pat. No. 4,035,565 and U.S. Pat. No. 4,694,045), or iodo and/or bromo fluoroalkylvinylethers (as described in U.S. Pat. No. 4,745,165, U.S. Pat. No. 4,564,662 and EP 199,138), in such amounts whereby the content of xe2x80x9ccure-sitesxe2x80x9d comonomers in the final product is generally in the range 0.05-2 moles for 100 moles of the other base monomeric units.
Alternatively or also in combination with the xe2x80x9ccure-sitexe2x80x9d comonomers, it is possible to introduce in the end groups iodine and/or bromine atoms by addition to the reaction mixture of iodinated and/or brominated chain transfer agents, such as for example the compounds of formula Rf(I)x(Br)y, wherein Rf is a (per)fluoroalkyl or a (per)fluorochloroalkyl having from 1 to 8 carbon atoms, while x and y are integers in the range 0-2, with 1xe2x89xa6x+yxe2x89xa62 (see for example U.S. Pat. No. 4,243,770 and U.S. Pat. No. 4,943,622). It is also possible to use as chain transfer agents iodides and/or bromides of alkaline or alkaline-earth metals, according to U.S. Pat. No. 5,173,553.
Preferably the crosslinking of radical type uses ionomers containing units of the bis-olefin of formula (I) and iodine in terminal position.
The sulphonic ionomer of the invention is crosslinked by radical route at a temperature in the range 100xc2x0 C.-200xc2x0 C., depending on the type of type used peroxide, by addition of a suitable peroxide capable to generate radicals by heating. Generally, the peroxide amount is in the range 0.1%-5% by weight with respect to the polymer. Among them it can be mentioned: dialkylperoxides, such as for example di-terbutyl-peroxide and 2,5-dimethyl-2,5-di(terbutylperoxy)hexane; dicumyl peroxide; dibenzoyl peroxide; diterbutyl perbenzoate; di-1,3-dimethyl-3-(terbutylperoxy)butylcarbonate. Other peroxidic systems are described, for example in the patent applications EP 136,596 and EP 410,351.
Besides it can be added before the crosslinking:
(a) a crosslinking co-agent, in an amount in the range 0.5-10%, preferably 1-7% by weight with respect to the polymer; among them it can be mentioned: triallyl-cyanurate; triallyl-isocyanurate (TAIC); tris(diallylamine)-s-triazine; triallylphosphite; N,N-diallyl-acrylamide; N,N,Nxe2x80x2,Nxe2x80x2-tetrallyl-malonamide; trivinyl-isocyanurate; 2,4,6-trivinyl-methyltrisiloxane; N,Nxe2x80x2bisallylbicyclo-oct-7-ene-disuccinimide (BOSA); bis olefin of formula (I), triazine;
(b) a metal compound, in amounts in the range 1-15%, preferably 2-10%, by weight with respect to the polymer, selected from oxides or hydroxides of divalent metals, such as for example Mg, Zn, Ca or Pb, optionally combined with a weak acid salt, such as for example stearates, benzoates, carbonates, oxalates or phosphites of Ba, Na, K, Pb, Ca;
(c) other conventional additives, such as thickeners, pigments, antioxidants, stabilizers and the like;
(d) inorganic or polymer reinforcing fillers, preferably optionally fibrillable PTFE. Preferably fillers have a size from 10 to 100 nm, preferably 10-60 nm.
A further object of the present invention is that the sulphonic ionomer can be mixed with a fluoroelastomer, preferably perfluoroelastomer, co-curable with the sulphonic ionomer of the invention. Preferably for the co-curing the fluoroelastomer contains iodine and/or bromine atoms. A TFE-/perfluoromethylvinylether copolymer having a ratio by moles in the range 80:20-60:40 of the type described in EP 661,304 in an amount comprised between 0-50% by weight with respect to the polymer, can for example be mentioned.
The ionomer and fluoroelastomer mixture can for example be a physical blend of solid polymers or of polymerization latexes. In this case the percentages of peroxides are to be referred to the ionomer and fluoroelastomer mixture. Also for the optional agents the percentages are to be referred to said mixture.
The crosslinking blend is prepared by using mechanical mixers.
The sulphonic fluorinated ionomers of the invention can be used for the preparation both of self-supported membranes and membranes supported on suitable support.
The self-supported membranes are obtained by subjecting the blend to a molding, extrusion or calendering process to obtain a film of the desired thickness, at a temperature lower than or equal to the temperature at which crosslinking takes place. When the film is obtained at a temperature lower than the crosslinking temperature, a thermal treatment is necessary to complete the crosslinking.
When the membranes are supported, as support, a fluorinated, preferably perfluorinated, porous material, having the desired size and thickness, can be used. In this case the sulphonic ionomer under the form of latex or of solution is crosslinked as above indicated, after being deposited on the support.
The crosslinked membranes of the invention are subjected to the activation treatment to transform the sulphonyl groups xe2x80x94SO2F into sulphonic groups xe2x80x94SO3H. For example activation can be carried out in 2 steps:
salification to transform the xe2x80x94SO2F form into the xe2x80x94SO3K form;
acidification to transform the xe2x80x94SO3K form into the xe2x80x94SO3H form.
For example the salification is carried out by immersing the membrane obtained after the crosslinking reaction in an aqueous solution containing 10% by weight of KOH at a temperature in the range 60xc2x0 C.-80xc2x0 C. for a time higher than 2 hours. When the salification is over, the membrane is immersed into a distilled water bath at room temperature to wash the residual KOH. THe acidification is carried out for example by placing the salified membrane in an aqueous solution containing 20% by weight of HCl at room temperature for at least 2 hours.
The resulting membrane in the xe2x80x94SO3H form is suitable to be used in fuel cell applications.
The membranes obtained with ionomers having a low equivalent weight (lower than 750 g/eq) show a high hydration percentage. However this high hydration percentage does not compromise the substantial physical integrity of the membrane. Indeed by immersing the crosslinked membrane of the invention at 100xc2x0 C. in water the membrane maintains integrity. On the contrary the uncrosslinked membranes subjected to the same treatment disintegrate or dissolve thus losing any physical integrity (see the Examples).
Apart from the preparation of membranes for fuel cells, the sulphonic ionomers of the present invention can successfully be used in the preparation of membranes used in electrochemical applications, such as for example chloro-alkali cells, lithium batteries, and electrodialysis and in reactors in which the ionomeric membrane acts as a superacid catalyst.
With the crosslinking system of the present invention, the crosslinking does not involve the sulphonyl groups xe2x80x94SO2F of the various polymer chains. In this way there is no reduction of the sulphonyl groups xe2x80x94SO2F available for the conversion into the sulphonic groups xe2x80x94SO3H. Consequently, with the crosslinking of the present invention there is not the drawback of a reduction of the sulphonic groups with consequent increase of the equivalent weight and consequent lowering of the ionic conductivity.
After the crosslinking step, the iodine, when present, can optionally be eliminated by a thermal post-treatment. Said post-treatment is carried out at a temperature preferably in the range 200xc2x0 C.-250xc2x0 C., temperature at which the break of the Cxe2x80x94I bonds takes place with consequent iodine elimination.
The polymerization of the monomers can be carried out in aqueous emulsion according to well known methods of the prior art, in the presence of radical initiators (for example alkaline or ammonium persulphates, perphosphates, perborates or percarbonates), optionally in combination with ferrous, cuprous or silver salts, or other easily oxidizable metals. In the reaction medium also surfactants of various type are usually present, among which the fluorinated surfactants of formula:
Rfxe2x80x94Xxe2x88x92M+
are particularly preferred, wherein Rf is a C5-C16 (per)fluoro-alkyl chain or a (per)fluoropolyoxyalkylene chain, Xxe2x88x92 is xe2x80x94COOxe2x88x92 or xe2x80x94SO3xe2x88x92, M+ is selected from: H+, NH4+, alkaline metal ion. Among the most commonly used we remember: ammonium perfluoro-octanoate, (per)fluoropolyoxyalkylenes ended with one or more carboxyl groups, etc.
When the polymerization is over, the ionomer is isolated by conventional methods, such as coagulation by addition of electrolytes or by cooling.
Alternatively, the polymerization reaction can be carried out in bulk or in suspension, in an organic liquid wherein a suitable radical initiator is present, according to well known techniques.
The polymerization reaction is generally carried out at temperatures in the range 25xc2x0-120xc2x0 C., under pressure up to 3 MPa.
The preparation of the sulphonic ionomers of the present invention is preferably carried out by using a dispersion or microemulsion of perfluoropolyoxyalkylenes, according to U.S. Pat. No. 4,789,717 and U.S. Pat. No. 4,864,006.
The present invention will be now better illustrated by the following embodiment Examples, which have a merely indicative but not limitative purpose of the scope of the invention itself.